Anyone know what this code is doing?

In an exercise in my book, I've come across an example that doesn't go too far into explaining a section of the code. I don't know what it's called so I'm having a tough time finding it. Here's a clip of it, what I've highlighted in bold and underlined is what's got me stumped...

Code :

---

public class Sales2
{
   public static void main( String[] args )
   {
      Scanner input = new Scanner( System.in );
      // sales array holds data on number of each product sold
      // by each salesperson
      double[][] sales = new double[ 5 ][ 4 ];
 
      System.out.print( "Enter salesperson number (-1 to end): " );
      int person = input.nextInt();
 
      while ( person != -1 )
      {
         System.out.print( "Enter product number: " ); //
         int product = input.nextInt();
         System.out.print( "Enter sales amount: " );
         double amount = input.nextDouble();
 
         // error-check the input
         if ( person >= 1 && person < 5 &&
               product >= 1 && product < 6 && amount >= 0 )
           [B][U][SIZE="3"] sales[ product - 1 ][ person - 1 ] += amount;[/SIZE][/U][/B]
         else
            System.out.println( "Invalid input!" );
 
         System.out.print( "Enter salesperson number (-1 to end): " );
         person = input.nextInt();
      }

---

Any idea what "sales[ product - 1 ][ person - 1 ] += amount;" is doing? Because I don't, I'm stumped. Anyone know what it is called at the very least? :confused: [-O<

Wow, I'm dumb, Java = Zero based, so sales[ product -1] [ person -1 ] simply sets the value entered to the correct element in the array... Right?

I think that's right at least.

You're right, of course. The people and products are indexed from 1 to 4, and the products 1 to 5, so subtracting by 1 allows these indices to be used for the arrays which are indexed from 0 to 3 and from 0 to 4, respectively.